Atmospheric pollutant sensing device

ABSTRACT

It has been found that a laser can provide emissions which are absorbed by pollutants, such as NO, NO2, N2O4, SO2 and CO. The laser emission is used to resonantly excite the pollutant molecules and then to act as a local oscillator for a superheterodyne radiometer to detect the fluorescence occurring in response to the stimulation.

United States Patent Menzies Sept. 25, 1973 ATMOSPHERIC POLLUTANTSENSING 3,566,l14 4/1968 Brewer 250/71 R DEVICE OTHER PUBLICATIONS [75]Inventor: Robert T. Menzies, Pasadena, Calif. [73] Assignee: CaliforniaInstitute of Technology, i j Heterodyne Detecnon of a Weak LightPasadena Cam- Beam; J. Opt. Soc.; Vol. 56, No. 9; Sept. 66; pp.

l200-l206; Sci. Lib. [22] Filed: Apr. 21,1971

[ pp 135,929 Primary ExaminerArchie R. Borchelt Related US ApplicationData Attorney-Lindenberg, Freilich & Wasserman [63] Continuation-impartof Ser. No. 80,583, Oct. 14,

1970, abandoned.

[57] ABSTRACT 2 P' 150/338 5f9 i33 It has been found that a laser canprovide emissions g H 5 R which are absorbed by pollutants, such as NO,N0 1 i s g R 71 5 N 0 S0 and CO. The laser emission is used to reso- 96nantly excite the pollutant molecules and then to act as a localoscillator for a superheterodyne radiometer to detect the fluorescenceoccurring in response to the [56] References Cited 7 stimulation. UNITEDSTATES PATENTS 3.446.558 5/1969 Seaton 250/435 R 13 Claims, 4 DrawingFigures l2 f 50 -\l 54 i l OP 58 m v M 36 9.4 42 l O I R F R F DETECTORAMP L lNDlCATOl? 1 ATMOSPHERIC POLLUTANT SENSING DEVICE CROSS-REFERENCETO RELATED APPLICATIONS This application is a continuation-in-part ofapplication Ser. No. 80,583 filed Oct. 14, 1970, for ATMO- SPHERICPOLLUTANT SENSING DEVICE, and now abandoned.

The invention herein described was made in the 1 course of or under agrant with the United States Air Force Office of Scientific Research,Office of Aerospace Research, U.S. Air Force.

BACKGROUND OF THE INVENTION This invention relates to radiometerdetectors and more particularly to improvements therein.

There is presently a great deal of concern over the pollutants which arefilling the earth s atmosphere. One of the pollutants which occurs whenfossile fuels are burned is nitric oxide. This is found in the emissionsfrom smoke stacks as well as automobile exhausts and is involved in thechemical process forming smog. A great deal of effort has gone intodeveloping instruments which can observe the thermal emissions, chimneysor exhausts, etc. and can indicate whether or not they contain thesesmog producing molecules.

OBJECTS AND SUMMARY OF THE INVENTION An object of this invention is toproduce a radiometer detector which is operative in response tofluorescence of an impurity as nitric oxide, when stimulated by laserradiation.

Another object of this invention is the provision of a novel laserradiometer detector. Yet, another object-of the present invention is theprovision of a nitric oxide radiometer detector.

These and other objects of this invention are achieved, in oneembodiment of this invention, by directing laser radiations, havingwavelengths which are absorbed by the pollutants, at the source ofpollutants. The light stimulated pollutants emit characteristicradiations which may be a heterodyned with other laser radiations toproduce signals indicative of the presence and quantity of pollutants.In a second embodiment of the invention a second laser may be used toprovide signals of the desired heterodyning frequency. The laser may bea C laser or a C0 laser, the frequency of whose output may be doubled.Pollutants such as S0 NO, N0 N 0 and C0 are thus detectable.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will best be understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of anembodiment of the invention.

FIG. 2 is a schematic diagram of another embodiment of the invention.

FIG. 3 is a partial view of the invention showing a modificationthereof, and

FIG. 4 is a schematic diagram of still another embodiment of theinvention.

Several spectral coincidences in the 5 to 6 micron wavelength regionbetween emission lines of the carbon monoxide laser and absorption linesof NO and N 0,, have been observed. Also, it has been observed that S0absorbs wavelengths in the region of 7 to 8 microns, which are emittedby the C0 laser. In addition,

0 coincidences with absorption lines of N0 and CO can be predicted onthe basis of laboratory measurements.

invention to be described.

FIG. 1 is a block schematic diagram of a radiometer for detecting thepresence of pollutants, such as those mentioned, wherein the pollutantsare emitted from a source, such as a smoke stack 10. The smoke stackemission 12 contains thermal radiation from the emitted particles andmolecules such as thermal radiation from NO. The radiometer includes aC0 laser, 14, which acts as a local oscillator. It emits severaldifferent lines, some of which coincide with the NO or other mentionedpollutant thermal emission lines in their wavelengths. The light emittedby the C0 laser is directed, by a beam splitter 16, at a detector 18.

Some of the thermal radiation emitted by NO will also pass through thebeam splitter 16 (signified by wavy arrow) and fall on detector 18. TheCO laser line which closely coincides with an NO thermal emission linein wavelength will produce a strong heterodyne signal from the detector18. The detector may be a photoconductor or photodiode made of leadselenide or indium antimonide. These photoconductors are sensitive to a5 micron wavelength which is in the infrared region. The detectionsystem is responsive only to a signal produced as a result ofheterodyning the C0 laser emission and that part of the NO emissionwhich is of very nearly the same wavelength. The output of the detectorelement, consisting of the heterodyned two input frequencies will beapplied to an R.F. amplifier 22, the output of which is applied to anR.F. detector 24. The detector produces an output having an amplitudeindicative of the amount of NO present. The output of the RF. detectoris applied to an indicator 26, which may be a low' pass filter and ameter indicating the amount of the NO present. This is determined by theamplitude of the signal produced by the detector 18 in response to theproduct term of the two heterodyned signals.

This will be understood from the following. If it is assumed that theradiation from the laser is 1 cosine w T and a part of the radiationfrom the NO particle is I cosine w, T, then the detector response willbe 1 I l l cosine (m -co on the assumption that ru -m is small enough sothat the detector will respond to the product term l l cosine (m -m FIG.2 is a preferred embodiment of the invention. Here, the same C0 laseremits certain lines which, by means of a prism 30, and a mirror 32, aredirected at the emission plume 12. These lines are the ones with whichare coincident in wavelength with resonant absorption and emission linesof NO, for example or of any of the other mentioned pollutants. Theseare designated as A For example one value of A could be 5.1666 microns,which is equivalent to 1935.48 cm in wavenumber units.

Other lines in the emission from the C0 laser which are slightlydifferent than 1. and are designated as M are directed at a secondmirror 34. It is important to note that both A and A coincide closelywith absorption and emission lines of NO, or some other pollutantmolecule of interest. This mirror directs the line A at a thirdreflector 36, which is a beam splitter. The line A can equal, forexample, 5.1886 microns, or 1927.28 cm.

The laser lines which are directed at the plume and which are coincidentin wavelength with some of the resonant absorption and emission lines ofthe NO will excite the N molecules to a high vibrational energy level.These molecules would then collide with nearby molecules and distributethemselves among the rotational levels of the higher vibrational level.

As a consequence they will emit characteristic radiation in certainnarrow wavelength bands, which are referred to as emission offluorescent lines, and which are characteristic of the NO molecule.These are the same wavelengths as the NO thermal emission linespreviously mentioned. These wavelengths are not necessarily the samewavelenths as that of the original exciting beam of radiation. Thisradiation passes through the beam splitter 36 and, together with theradiation A falls upon the detector 38. The detector is the same as isdescribed in connection with FIG. 1. The detector will respond toradiation from that part of an NO emission line which is within around 1gigahertz (GI-Iz) away from A in frequency units. The output of thedetector is applied to an R.F. amplifier 42. The output of the R.F.amplifier to an RF. detector 44. The output of the R.F. detector isapplied to an indicator 46. The operation of these circuits is aspreviously described. The indicator is only sensitive to the productterm achieved by heterodyning M with the fluorescent radiation frequencyof the NO.

FIG. 3 is a view illustrating another arrangement for this invention.Only so much of the invention is shown as differs from the views shownin FIGS. 1 and 2. The laser 50 is a C0 laser. Its output frequency isdoubled by passing its output through a non-linear crystal 52 which hasno inversion symmetry and which transmits wavelengths in the 4.5 to 12micron region. Some examples of materials suitable for such a crystalare materials such as tellurium, proustite or pyrargyrite.

The frequency-doubled CO laser output is next passed through a beamsplitter 54 which directs a portion of the light at a detector 56 andthe balance passes through the beam splitter to a mirror 58 whichdirects the light toward a source of emissions to be tested. The lightfrom the fluorescing pollutants passes to the detector 56. The frequencydoubled laser beam can provide infrared emissions of various wavelengthswhich are absorbed by pollutants such as NO, N0 N 0 and CO.

FIG. 4 represents an alternate arrangement to that previously shown anddescribed. A laser 60, is pulsed or Q switched, emitting radiation ofwavelength A which is directed by a mirror 62 at the smoke plume 64, orregion of air to be monitored. The radiations returned by the smokeplume may be at wavelengths A A or A, A second laser 66, which is acontinuous laser, emits radiation at a wavelength A and has itscontinuous light output directed by a beam splitter 68 at a detector 70.The light returned by the fluorescing contaminants in the smoke plumepasses through the beam splitter to the detector 70, there to behexterodyned with the light from laser 66. The heterdyned output of thedetector is applied to the RF. amplifier 72, thereafter to the R.F.detector 74, whose output is applied to an indicator 76.

The laser and 66 may be CO lasers, CO lasers, or CO2 lasers whose outputfrequency is doubled by using a crystal such as crystal 52 shown in FIG.3.

If desired a Dicke radiometer configuration may be used for theheterodyning receiver shown. A Dicke radiometer is one wherein arotating blade is placed in front of the detector, which interrupts theradiation incident on the receiver at a regulator interval. The rotationfrequency may be within a range of lO-l0,000 cycles/sec. The indicatorincludes a lock-in amplifier which responds only to the portion of itsinput whose frequency and phase are determined by the chopping frequencyof the rotating blade. The lock-in amplifier is an amplifier whose inputis tuned to respond to frequency and phase to an input occurring at thefrequency and phase of the rotating blade. The amplifier output isfollowed by an integrator. The integrator output may drive a meter.

It will be appreciated that the radiometers described above need not bein contact with the emissions desired to be inspected but can bepositioned remotely therefrom. Prior art detectors normally require asample to be placed within an instrument for spectral or chemicalanalysis therein. In view of the use of this invention of the phenomenomof resonant excitation, an instrument, in accordance with this inventioncan be very discriminating and sensitive.

There has accordingly been described hereinabove a novel, and usefulradiometer. It will be appreciated that while the description has beengiven for detecting (NO) particles such as NO, N0 N 0,, CO and S0 thisis by way of illustration and not to be construed as a limitation uponthe invention.

What is claimed is:

l. A radiometer for detecting radiations from a chemical compound in anemission comprising:

laser means for producing light radiations the wavelengths of which arenearly coincident with the wavelengths of the radiations from saidchemical compound,

detector means positioned to receive the radiations from said chemicalcompound and said laser means for heterodyning said radiations andproducing a resultant output, and

indicator means responsive to said resultant output for indicating thepresence and amount of said chemical compound.

2. A radiometer as recited in claim 1 wherein said laser means is acarbon monoxide laser means and said chemical compound radiations arethermal radiations.

3. A radiometer as recited in claim 1 wherein said laser means produceslight having wavelengths which when directed at said emission causessaid chemical compound to produce fluorescent radiations, and

means for directing said laser means light at said emission.

4. A radiometer as recited in claim 3 wherein said laser means is oneselected from a group consisting of a C0 laser means or a C0 lasermeans.

5. A radiometer as recited in claim 4 wherein said CO laser meansincludes means for doubling the frequency of the light output of said COlaser means.

6. A radiometer for detecting the presence of a chemical compound in anemission comprising:

laser means for producing light having a frequency which which directedat said emission causes said chemical compound to produce fluorescentradiation,

detector means for producing an output responsive to the combination ofsaid laser means emission and said chemical compound fluorescentradiation being applied thereto, 7 means for directing light from saidlaser means at said detector means and at said emission causing saidchemical compound to produce fluorescent radiation some of which fallson said detector means,

and I an indicator for indicating the amplitude of said detector meansoutput.

7. A radiometer as recited in claim 6 wherein said laser means comprisesa carbon dioxide laser and means for doubling the frequency of the lightoutput of carbon dioxide laser 8. A radiometer as recited in claim 6wherein said laser means includes a first and second laser,

said means for directing light from said laser means at said detectormeans and at said emission includes means for directing light from saidfirst laser means at said emission, and

means for directing light from said second laser means at said detectorelement.

9. A radiometer as recited in claim 6 wherein said means for directingsaid laser light includes means for directing a portion of the lightoutput of said laser means at said emission to cause fluorescense ofsaid chemical compound some of said fluorescent radiation travellingback to said detector means, and

means for directing another portion of said laser light at said detectormeans, which responds to the combination of said laser light radiationand said fluorescent radiation of said chemical compound.

10. A radiometer for detecting the presence of a chemical compound in anemission comprising:

laser means for producing light having a frequency which when directedat said emission causes said chemical compound to produce fluorescentradiations,

photodetector means for converting the light of said laser means andsaid fluorescent radiations to electrical signals, heterodyning saidsignals and producing an output representative thereof,

' means for directing light from said laser means at said detectorelement and at said emission whereby some of the fluorescent radiationsproduced by said chemical compound fall on said photodetector means, and

an indicator for indicating responsive to said photodetector meansoutput the presence and amount of said chemical compound in saidemission. 11. A radiometer as recited in claim 10 wherein said lasermeans comprises a carbon dioxide laser and means for doubling thefrequency of the light output of said carbon dioxide laser. 12. Aradiometer as recited in claim 10 wherein said laser means includes afirst and second laser,

said means for directing light from said laser means at said detectormeans and at said emission includes means for directing light from saidfirst laser means at said emission, and means for directing light fromsaid second laser means at said detector element. 13. The method ofdetecting the presence of a chemical compound in a gaseous emissioncomprising:

generating a laser light beam having a frequency which when applied tosaid gaseous emission causes the chemical compound to producefluorescent radiation, directing said laser light beam, at said gaseousemission, combining some of said fluorescent radiation with some of saidlaser light beam to produce a difference signal, and displaying saiddifference signal to indicate the presence of said chemical compound insaid gaseous emission.

2. A radiometer as recited in claim 1 wherein said laser means is acarbon monoxide laser means and said chemical compound radiations arethermal radiations.
 3. A radiometer as recited in claim 1 wherein saidlaser means produces light having wavelengths which when directed atsaid emission causes said chemical compound to produce fluorescentradiations, and means for directing said laser means light at saidemission.
 4. A radiometer as recited in claim 3 wherein said laser meansis one selected from a group consisting of a CO laser means or a CO2laser means.
 5. A radiometer as recited in claim 4 wherein said CO2laser means includes means for doubling the frequency of the lightoutput of said CO2 laser means.
 6. A radiometer for detecting thepresence of a chemical compound in an emission comprising: laser meansfor producing light having a frequency which which directed at saidemission causes said chemical compound to produce fluorescent radiation,detector means for producing an output responsive to the combination ofsaid laser means emission and said chemical compound fluorescentradiation being applied thereto, means for directing light from saidlaser means at said detector means and at said emission causing saidchemical compound to produce fluorescent radiation some of which fallson said detector means, and an indicator for indicating the amplitude ofsaid detector means output.
 7. A radiometer as recited in claim 6wherein said laser means comprises a carbon dioxide laser and means fordoubling the frequency of the light output of carbon dioxide laser.
 8. Aradiometer as recited in claim 6 wherein said laser means includes afirst and second laser, said means for directing light from said lasermeans at said detector means and at said emission includes means fordirecting light from said first laser means at said emission, and meansfor directing light from said second laser means at said detectorelement.
 9. A radiometer as recited in claim 6 wherein said means fordirectIng said laser light includes means for directing a portion of thelight output of said laser means at said emission to cause fluorescenseof said chemical compound some of said fluorescent radiation travellingback to said detector means, and means for directing another portion ofsaid laser light at said detector means, which responds to thecombination of said laser light radiation and said fluorescent radiationof said chemical compound.
 10. A radiometer for detecting the presenceof a chemical compound in an emission comprising: laser means forproducing light having a frequency which when directed at said emissioncauses said chemical compound to produce fluorescent radiations,photodetector means for converting the light of said laser means andsaid fluorescent radiations to electrical signals, heterodyning saidsignals and producing an output representative thereof, means fordirecting light from said laser means at said detector element and atsaid emission whereby some of the fluorescent radiations produced bysaid chemical compound fall on said photodetector means, and anindicator for indicating responsive to said photodetector means outputthe presence and amount of said chemical compound in said emission. 11.A radiometer as recited in claim 10 wherein said laser means comprises acarbon dioxide laser and means for doubling the frequency of the lightoutput of said carbon dioxide laser.
 12. A radiometer as recited inclaim 10 wherein said laser means includes a first and second laser,said means for directing light from said laser means at said detectormeans and at said emission includes means for directing light from saidfirst laser means at said emission, and means for directing light fromsaid second laser means at said detector element.
 13. The method ofdetecting the presence of a chemical compound in a gaseous emissioncomprising: generating a laser light beam having a frequency which whenapplied to said gaseous emission causes the chemical compound to producefluorescent radiation, directing said laser light beam, at said gaseousemission, combining some of said fluorescent radiation with some of saidlaser light beam to produce a difference signal, and displaying saiddifference signal to indicate the presence of said chemical compound insaid gaseous emission.